In general, liquefied natural gas (“LNG”) is obtained by causing natural gas, one of fossil fuels, to be liquefied. An LNG storage tank is classified into a ground storage tank, which is installed on the ground or buried in the ground according to installation positions, and a mobile storage tank, which is mounted on transportation means such as automobiles and ships.
The aforementioned LNG is stored in a cryogenic state and is explosive when it is exposed to shock. Thus, the LNG storage tank should be constructed such that shock resistance and liquid-tight characteristics thereof can be firmly maintained. The LNG storage tank installed on a mobile automobile or ship is slightly different from the ground storage tank with little motion in view of their configurations in that it should provide a means for overcoming mechanical stress due to the motion thereof. However, the LNG storage tank, which is installed on a ship and provided with a means for overcoming the mechanical stress, can also be used as a ground storage tank. Therefore, the structure of an LNG storage tank installed on a ship will be described herein by way of example.
First, an LNG storage tank installed within an LNG carrier may be classified into an independent tank type and a membrane type. This corresponds to classification according to whether cargo load is applied directly to an insulating material, and detailed description thereof will be discussed as follows.
As shown in Table 1, GT type made in Gaz Transport and TGZ type made in Technigaz are renamed and used as GTT NO 96-2 and GTT Mark III, respectively, as Gaz Transport (GT) and Technigaz (TGZ) are merged into and renamed as Gaztransport & Technigaz (GTT) in 1995.
The structures of the aforementioned GT type and TGZ type tanks are described in U.S. Pat. No. 6,035,795, U.S. Pat. No. 6,378,722, U.S. Pat. No. 5,586,513, U.S. Patent Laid-Open Publication No. 2003-0000949, Korean Patent Laid-Open Publication No. 2000-0011346, and the like.
TABLE 1Classification of LNG storage tanksMembrane TypeGTTGTTIndependent TypeItemMark IIINO 96-2MOSSIHI - SPBTankSUS 304LInvar SteelAl Alloy SteelAl Alloy SteelMaterialthickness 1.2 mm 0.7 mm 50 mmMax. 30 mmInsulatingReinforcedPlywoodPolyurethanePolyurethaneMaterialPolyurethaneBox +FoamFoamFoamPerlitethickness250 mm530 mm250 mm200 mm
The membrane type LNG carrier of GTT is configured in such a manner the cargo load is directly applied to an insulating material or ship's hull and a cofferdam is installed between adjacent cargo tanks to avoid danger due to mechanical/thermal characteristics. Further, an air temperature in the cofferdam should be kept at a temperature of +5° C. or more in order to prevent low-temperature brittleness in an inner plate at a side of the cofferdam. To this end, a heating means such as a heating coil is generally installed to utilize a heat source such as steam or hot water. In order to construct the insulating material, a scaffold is first installed at a ship's hull, and scaffold materials, insulation boxes and membranes manufactured on land, and other materials are then carried and installed. A working hour before launch is longer in case of an old tank, whereas a working hour after launch is longer in a membrane type.
As shown in FIGS. 1 and 2, a GTT NO 96-2 type carrier among the GTT membrane type carriers is made of Invar steel (36% Ni) with a thickness of 0.5˜0.7 mm, and first and second sealing barriers 10 and 15 have the almost same liquid-tight characteristics and strength as each other. Therefore, cargo can be safely carried using only the second sealing barrier 15 in a substantial period of time even when the first sealing barrier 10 leaks. Further, since a membrane of the sealing barriers 10 and 15 of the GTT NO 96-2 is straight, it can be more conveniently welded than a Mark III type corrugated membrane. Accordingly, the automation ratio of GTT NO 96-2 type is higher than that of GTT Mark III type, whereas the overall length of GTT NO 96-2 type to be welded is longer than that of GTT Mark III type.
Furthermore, the currently employed GTT NO 96-2 type is most different from the conventional GT type in that instead of U-shaped bars, a plurality of double couples 17 are used to support the insulation box 11 and 16 (insulation barrier). The functions of main parts of heat-insulating sections of the GTT NO 96-2 type storage tank of the LNG carrier are as shown in Table 2.
TABLE 2Main parts of heat-insulating sections ofGTT NO 96-2 type storage tankItemFunctionTongueIt is installed at an insulation box and weldedin three-ply way between membrane sheets toconnect them, and it allows the membrane andinsulation box to be connected to each other.JoistIt is installed between the insulation boxes toreduce horizontal displacement and preventhigh stress from being created.First and secondIt prevents heat from being transferred into theinsulation barriersstorage tank.(Perlite)First sealing barrierIt provides a primary countermeasure and is a(Invar)portion that comes into direct contact with thecargo having a temperature of −163° C. andprimarily defines the storage tank.Second sealing barrierIt provides a secondary countermeasure and(Invar)performs a function of preventing cargo fromleaking out during a predetermined period oftime when the first sealing barrier is brokendown.
On the other hand, as shown in FIGS. 3 and 4, a GTT Mark III type is made of a stainless steel membrane with waveforms having a thickness of 1.2 mm, as a first sealing barrier 20, attached thereto. In such a case, since contraction due to low temperature is absorbed in folds of the corrugations, large stress is hardly created in the membrane. Further, insulation barriers 21 and 26 are made of polyurethane foam, glass wool, Triplex or the like. The Mark III type is constructed in such a manner that the first and second insulation barriers 21 and 26 are manufactured on land and then integrated thereto. Therefore, the construction of the Mark III type is relatively easy as compared with the GTT NO 96-2 type in which the first and second insulation boxes 21 and 26 are respectively installed.
The functions of main parts of heat-insulating sections of the GTT Mark III type storage tank of the LNG carrier are as shown in Table 3.
TABLE 3Main parts of heat-insulating sections ofGTT Mark III type storage tankItemFunctionMasticIt transfers cargo load to the ship's hull.PlywoodIt is installed between the first and secondsealing barriers and the first and secondinsulation barriers, allows constant load to beapplied to the sealing barriers due to theuniform arrangement of the insulation barriers,and reduces the displacement created due tovertical load.Glass woolIt is installed between the insulation boxes,reduces the horizontal displacement andprevents the occurrence of high stress.First and secondIt prevents heat from being transferred into theinsulation barriersstorage tank.(Polyurethane foam)Second sealing barrierIt has a function of preventing the cargo from(Triplex)leaking out during a predetermined period oftime when the first membrane, i.e. the firstsealing barrier, and is configured such that theglass cloth is bonded to both surfaces of Alfoils.First sealing barrierIt is a first membrane with which cargo with a(SUS 304L)temperature of about −163° C. is brought intocontact, primarily defines a cargo tank, and isconstructed to have such a corrugated structurethat it can withstand thermal stress.
An important part of the GTT NO 96-2 type and GTT Mark III type storage tanks so configured is a corner part.
Here, the corner part (edge part) of the LNG storage tank is a region to which load created due to thermal stress of the respective sealing barriers (membranes) of the storage tank is asymmetrically applied. This corner part should be constructed such that the stress created from the storage tank can be eliminated by distributing the asymmetrical load.
A recent technology for the corner part (edge part) of the LNG storage tank includes “a water-tight and thermally insulating tank with an improved corner structure, built into the bearing structure of a ship” described in Korean Patent Laid-Open Publication No.2000-0011347.
As shown in FIG. 5, the corner structure disclosed in the above Korean publication No. 2000-0011347 causes a prefabricated composite girder 30 to be fixed at a right angled region where a cross bulkhead 2 and an inner face 1 of the ship's hull join together. The composite bulkhead 30 comprises a heat-insulating material 40 including reinforced webs 39 (shown in a dotted line) that are formed at a regular interval on a hard W-shaped metal body 31.
This type of prefabricated composite girder 30 is configured in such a manner that portions brought into surface contact with the cross bulkhead 2 and inner face 1 of the ship's hull are fixed thereto via polymeric resin 34 and opposite branched surfaces are mechanically fastened to the bearing structure of the ship's hull by means of fixing means 32 and 33 that are supported on the cross bulkhead 2 and inner face 1 of the hull, respectively.
In addition, a bottom surface of the prefabricated composite girder 30 has an inclined surface 42 such that a drainage space 41 is formed at the right-angled portion where the inner face 1 and cross bulkhead 2 join together.
The technology for fabricating the corner part of the LNG storage tank using the aforementioned prefabricated composite girder 30 has advantages in that the installation costs become inexpensive thanks to its simple structure and resistance of the sealing barriers against the mechanical impact can be improved without impairing the painted portion of the double bulkhead. However, the fabricating process for the corner part is not easy because the prefabricated composite girder 30, i.e. a basic unit of the corner part of the storage tank, includes the hard metal body 31 which in turn is manually fixed to the cross bulkhead 2 and inner face 1 of the hull by means of mechanical fixing means 32 and 33 (e.g., bolts and nuts) fixedly formed on the bulkhead and inner face.
The corner structure of the aforementioned membrane type LNG storage tank is a structure where the prefabricated composite girder 30, i.e. the basic unit of the corner part of the aforementioned storage tank, is firmly fixed to the cross bulkhead 2 and inner face 1 of the ship's hull. Therefore, any stress may be partially produced due to a wave or when the hull is moved, and thus, may be concentrated on the corner part. Accordingly, some efforts to reduce the stress concentration are made for several decades and continuous efforts to reduce the stress concentration are further needed.
Furthermore, continuous efforts to reduce boiled off gas (BOG), i.e. loss due to vaporization of cryogenic LNG, and to simplify the structure and manufacturing process of the LNG storage tank are further made.